This invention relates in general to electrophotographic imaging systems and, more specifically, to electrophotographic imaging members utilizing polymeric tertiary arylamine compounds.
In the art of electrophotography an electrophotographic plate comprising a photoconductive insulating layer on a conductive layer is imaged by first uniformly electrostatically charging the surface of the photoconductive insulating layer. The plate is then exposed to a pattern of activating electromagnetic radiation such as light, which selectively dissipates the charge in the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic toner particles on the surface of the photoconductive insulating layer. These toner particles may be applied to the surface in dry form or dispersed in a liquid carrier medium. The resulting visible toner image can be transferred to a suitable receiving member such as paper. This imaging process may be repeated many times with reusable photoconductive insulating layers.
As more advanced, higher speed electrophotographic copiers, duplicators and printers were developed, degradation of image quality was encountered during cycling. Moreover, complex, highly sophisticated, duplicating and printing systems operating at high speeds have placed stringent requirements including narrow operating limits on photoreceptors. For example, the numerous layers found in many modern photoconductive imaging members must be highly flexible, adhere well to to adjacent layers, be mechanically compatible for flexible belt photoreceptors, and exhibit predictable electrical characteristics within narrow operating limits to provide excellent toner images over many thousands of cycles. There is also a great current need for long service life, flexible photoreceptors in compact imaging machines that employ small diameter support rollers for photoreceptor belt systems compressed into a very confined space. Small diameter support rollers are also highly desirable for simple, reliable copy paper stripping systems which utilize the beam strength of the copy paper to automatically remove copy paper sheets from the surface of a photoreceptor belt after toner image transfer. However, small diameter rollers, e.g. less than about 0.75 inch (19 mm) diameter, raise the threshold of mechanical performance criteria for photoreceptors to such a high level that spontaneous photoreceptor belt material failure becomes a frequent event for flexible belt photoreceptors.
One type of multilayered photoreceptor that has been employed as a belt in electrophotographic imaging systems comprises a substrate, a conductive layer, a charge blocking layer a charge generating layer, and a charge transport layer. The charge transport layer often comprises an activating small molecule dispersed or dissolved in a polymeric film forming binder. Generally, the polymeric film forming binder in the transport layer is electrically inactive by itself and becomes electrically active when it contains the activating molecule. The expression "electrically active" means that the material is capable of supporting the injection of photogenerated charge carriers from the material in the charge generating layer and is capable of allowing the transport of these charge carriers through the electrically active layer in order to discharge a surface charge on the active layer. The multilayered type of photoreceptor may also comprise additional layers such as an anti-curl backing layer, an adhesive layer, and an overcoating layer. Adhesive layers are often required to overcome inadequate adhesion between substrate, blocking and generator layers. The elimination of this layer would remove a fabrication step, resulting in processing time savings, reduced costs and an increasing yield due to the decrease in handling. In addition it would eliminate potential electrical problems often associated with interfaces. Although excellent toner images may be obtained with multilayered belt photoreceptors that are developed with dry developer powder (toner), it has been found that these same photoreceptors become unstable when employed with liquid development systems. These photoreceptors suffer from cracking, crazing, delamination, crystallization of active compounds, phase separation of activating compounds and extraction of activating compounds caused by contact with the organic carrier fluid, isoparaffinic hydrocarbons e.g. Isopar.RTM., commonly employed in liquid developer inks which, in turn, markedly degrade the mechanical integrity and electrical properties of the photoreceptor. More specifically, the organic carrier fluid of a liquid developer tends to leach out activating small molecules, such as the arylamine containing compounds typically used in the charge transport layers. Representative of this class of materials are: N,N'-diphenyl-N,N'-bis(3-methylphenyl)-[1,1'-biphenyl]-4,4'-diamine; bis-(4-diethylamino-2-methylphenyl)-phenylmethane; 2,5-bis-(4'-dimethylaminophenyl)-1,3,4-oxadiazole; 1-phenyl-3-(4'-diethylaminostyryl)-5-(4"-diethylaminophenyl)pyrazoline; 1,1-bis-( 4-(di-N,N'-p-methylphenyl)-aminophenyl)-cyclohexane; 4-diethylaminobenzaldehyde-1,1-diphenylhydrazone; 1,1-diphenyl-2(p-N,N-diphenylaminophenyl)-ethylene; N-ethylcarbazole-3-carboxaldehyde-1-methyl-1-phenylhydrazone. The leaching process results in crystallization of the activating small molecules, such as the aforementioned arylamine compounds, onto the photoreceptor surface and subsequent migration of arylamines into the liquid developer ink. In addition, the ink vehicle, typically a C.sub.10 -C.sub.14 branched hydrocarbon, induces the formation of cracks and crazes in the photoreceptor surface. These effects lead to copy defects and shortened photoreceptor life. The degradation of the photoreceptor manifests itself as increased background and other printing defects. The ink vehicle and its vapor, may penetrate the photoreceptor layers exacerbating adhesion failures which lead to complete physical photoreceptor failure.
The leaching out of the activating small molecule also increases the susceptibility of the transport layer to solvent/stress cracking when the belt is parked over a belt support roller during periods of non-use. Some carrier fluids also promote phase separation of the activating small molecules, such as arylamine compounds and their aforementioned derivatives, in the transport layers, particularly when high concentrations of the arylamine compounds are present in the transport layer binder. Phase separation of activating small molecules also adversely alters the electrical and mechanical properties of a photoreceptor. Although flexing is normally not encountered with rigid, cylindrical, multilayered photoreceptors which utilize charge transport layers containing activating small molecules dispersed or dissolved in a polymeric film forming binder, electrical degradation is similarly encountered during development with liquid developers. In addition, even non-flexing photoreceptors can suffer from adhesion failure brought about by solvent penetration. Sufficient degradation of these photoreceptors by liquid developers can occur in less than eight hours of use thereby rendering the photoreceptor unsuitable for even low quality xerographic imaging purposes.
Photoreceptors have been developed which comprise charge transfer complexes prepared with polymeric molecules. For example, charge transport complexes formed with polyvinyl carbazole are disclosed in U.S. Pat. Nos. 4,047,948, 4,346,158 and 4,388,392. Photoreceptors utilizing polyvinyl carbazole layers, as compared with current photoreceptor requirements, exhibit relatively xerographic performance in both electrical and mechanical properties. Polymeric arylamine molecules prepared from the condensation of di-secondary amine with a di-iodo aryl compound are disclosed in European patent publication 34,425, published Aug. 26, 1981 and issued May 16, 1984. Since these polymers are extremely brittle and form films which are very susceptible to physical damage, their use in a flexible belt configuration is precluded. Thus, in advanced imaging systems utilizing multilayered belt photoreceptors exposed to liquid development systems, cracking and crazing have been encountered in critical charge transport layers during belt cycling. Cracks developing in charge transport layers during cycling can be manifested as print-out defects adversely affecting copy quality. Furthermore, cracks in the photoreceptor pick up toner particles which cannot be removed in the cleaning step and may be transferred to the background in subsequent prints. In addition, crack areas are subject to delamination when contacted with blade cleaning devices thus limiting the options in electrophotographic product design. A hole transporting polymer which produces an easy to clean surface would enable less aggressive cleaning systems to be used resulting in less wear, and hence longer life, of the photoreceptor.
Photoreceptors may need to have surfaces with special properties. The use of an overcoat has been one approach to achieve this. However, an adhesive layer has been found necessary for some overcoats to be mechanically functional. An adhesive layer, in addition to requiring an extra processing step, also introduces additional interfaces which may impact electrical performance. The use of an adhesion promoting transporting polymer for the transport layer can eliminate the need for an additional adhesive layer.
Photoreceptors having charge transport layers containing small molecule arylamine compounds dispersed or dissolved in various resins such as polycarbonates are known in the art. Similarly, photoreceptors utilizing polymeric arylamine containing molecules such as polyvinyl carbazole and polymethacrylates possessing pendant arylamines are also known. Further, condensation polymers of a di-secondary amine with a di-iodo aryl compound are described in the prior art. Moreover, various polymers derived from a reaction of certain monomers with aromatic amines such as N,N'-diphenyl-N,N'-bis(3-hydroxyphenyl)-[1,1'-biphenyl]-4,4'-diamine have recently been described.
Recently photoreceptors having charge transport layers containing charge transporting arylamine polymers have been described in the patent literature. These polymers include the products of a reaction involving a dihydroxy arylamine reactant and are described for example in U.S. Pat. Nos. 4,806,443, 4,806,443, 4,801,517 and 4,818,650, the entire disclosures of these patents being incorporated herein by reference. Although these polymers form excellent charge transport layers, many other polymeric derivatives of dihydroxy arylamines do not meet the numerous stringent requirements of sophisticated automatic electrophotographic systems. For example, the polymeric reaction products of dihydroxy arylamines and 1,3-diiodopropane form charge transport layers that possess very poor mechanical properties, are soft and non-robust and are of low molecular weight.